Pneumatic tire

ABSTRACT

A pneumatic tire comprises a tread-reinforcing band made of at least one spirally wound cord, wherein the band cord consists of materially identical high-strength high-modulus organic filaments, and the band cord is formed by first twisting each of filament bunches, subjecting the first twisted bunches to a dip-and-stretch treatment, and then twisting the treated bunches together, whereby the load-elongation curve of the band cord comprises a low-modulus zone and a high-modulus zone whose boundary point is in an elongation range of from 1 to 7%, and the ratio EH/EL of a nominal elastic modulus EH of the high-modulus zone and a nominal elastic modulus EL of the low-modulus zone is in a range of from 2.0 to 10.0. Therefore, high-modulus aramid cords, PEN cords and the like can be used to reduce the tire weight.

BACKGROUND OF THE INVENTION

The present invention relates to a pneumatic tire, more particularly toa tread-reinforcing band formed by spirally winding a specially treatedorganic fiber cord.

In order to improve the high-speed durability of a pneumatic tire suchas passenger car tire, motorcycle tire and the like, a tread reinforcingband made of a spirally wound cord is widely used in the tread portionalone or in combination with a breaker.

Usually, nylon fiber cords are used as the cords of such a treadreinforcing band because of the following reason. During vulcanizing agreen tire in a mold, the inside of the green tire is pressurized by theuse of an inflatable bladder to press the outer surface of the greentire onto the inner surface of the mold. Therefore, if the band is notstretched or the amount of stretch is insufficient, the tread rubber isnot fully pressed against the inner surface of the mold. As a result,the tire uniformity is deteriorated and vibrations are liable to occurduring running especially high speed running. Further, in the worstcase, molding defects occur in the tread portion. Accordingly, in orderto secure a sufficient stretch during vulcanization, nylon fiber cordswhich are relatively low in the modulus and thus stretchable duringvulcanization are widely used.

On the other hand, pneumatic tires are nowadays in urgent need ofeffective weight reductions to improve fuel economy or to reduce carbondioxide emissions.

In order to reduce the weight of a tread reinforcing band, if the lowmodulus nylon fiber cords used in a band is decreased in the numberand/or thickness, then it becomes difficult to secure the necessarystrength, especially the high-speed durability of the tire.

If high-strength cords such as aromatic polyamide fiber cords are usedas the band cords, the weight of the band can be reduced, but theabove-mentioned problems arise since the conventional high-strengthcords such as aromatic polyamide fiber cords have very high moduli andare hard to stretch during vulcanization as well as during use in thevulcanized tire.

SUMMARY OF THE INVENTION

It is therefore, an object of the present invention to provide apneumatic tire, in which the tread-reinforcing band is formed byspirally winding a specially treated organic fiber cord to enable aweight reduction of the band.

According to the present invention, a pneumatic tire comprises

a carcass extending between a pair of bead portions through a treadportion and a pair of sidewall portions, and

a band made of at least one spirally wound cord disposed radiallyoutside the carcass in the tread portion, wherein

the band cord consists of materially identical high-strengthhigh-modulus organic filaments, and the band cord is obtainable by thefollowing process (1) or (2):

(1) the high-modulus organic filaments are divided into a plurality ofbunches, the bunches are each first twisted separately from each other,the first twisted bunches are each subjected to a dip-and-stretchtreatment, and the treated bunches are second twisted together; or

(2) the high-modulus organic filaments are divided into a plurality ofbunches, the bunches are each first twisted separately from each other,the first twisted bunches are each subjected to a dip-and-stretchtreatment, some of the treated bunches are second twisted together toform a core, and at least one remaining treated bunch is twisted aroundthe core, whereby the band cord has a load-elongation curve comprising alow-modulus zone between the origin and a boundary point and ahigh-modulus zone between the boundary point and the cord breakingpoint, and the boundary point is in an elongation range between 1% and7%, and the ratio EH/EL of a nominal elastic modulus EH of thehigh-modulus zone and a nominal elastic modulus EL of the low-moduluszone is in a range of from 2.0 to 10.0.

As shown in FIG. 1, the boundary point P is defined as a point on theload-elongation curve J of which elongation percentage is the same asthat of the intersecting point P0 between two tangential lines T1 and T2to the load-elongation curve J at the original point (elongation=0) andthe breaking point P1 of the cord.

The nominal elastic modulus EL of the low-modulus zone S1 is defined bythe slope of a line drawn between the origin O and the boundary point P.The nominal elastic modulus EH of the high-modulus zone s2 is defined bythe slope of a line drawn between the boundary point P and breakingpoint P1.

The above-mentioned high-strength high-modulus organic filaments are,for example, aromatic polyamide filaments, fully aromatic polyesterfilaments, polyvinyl alcohol filaments whose strength is not less than15 g/dtex, carbon filaments, polyketone filaments, rayon filaments andthe like.

As described above, the band cord is formed by first twisting each ofthe filament bunches, and subjecting the first-twisted bunches to adip-and-stretch treatment, and then second twisting the treated bunchestogether. By making these processes in this order, the cord is providedwith an elongation characteristic having a low-modulus zone and ahigh-modulus zone as shown in FIG. 1 in full line.

If the cord is formed in the following order [first twisting each of thefilament bunches, second twisting the first-twisted bunches togetherinto the cord, and then subjecting the second twisted bunches(namely thecord) to the dip-and-stretch treatment], then the load-elongation curvej becomes almost linear from the original point to the breaking point P1as shown in FIG. 1 in broken line and the cord is provided with acompletely different elongation characteristic.

In this invention, therefore, in spite of a cord made of high-strengthhigh-modulus organic filaments, a sufficient stretch can be obtainedduring tire vulcanization, and as a result, a deterioration in the tireuniformity can be avoided. Further, because of the high-strengthhigh-modulus organic filaments, the thickness of the cord can bedecreased while maintaining the necessary cord strength. Accordingly,the weight of the band can be decreased to achieve a tire weightreduction without sacrificing the high-speed durability and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a load-elongation curve (full line) of a bandcord according to the present invention in contradistinction to aconventional load-elongation curve (broken line).

FIG. 2 is a cross sectional view showing a pneumatic tire according tothe present invention.

FIG. 3 is a schematic perspective view showing band cords rubberized ina form of a tape.

FIG. 4 is a schematic perspective view showing an example of the bandcord.

FIG. 5 is a schematic perspective view showing another example of theband cord.

FIG. 6 is a diagram for explaining a dip-and-stretch treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment of the present invention will now be described in detail inconjunction with accompanying drawings.

According to the present invention, pneumatic tire 1 comprises a treadportion 2, a pair of sidewall portions 3, a pair of bead portions 4 eachwith a bead core 5 therein, a carcass 6 extended between the beadportions 4 through the tread portion 2 and sidewall portions 3, and aband 9 disposed radially outside the crown portion of the carcass 6 inthe tread portion 2.

In this embodiment, the tire 1 is a radial tire for passenger carsprovided in the tread portion 2 with a breaker 7, and the band 9 isdisposed on the radially outside of the breaker 7 as shown in FIG. 2.

The carcass 6 is composed of at least one ply 6A of cords arrangedradially at an angle of 75 to 90 degrees with respect to the tireequator.

For the carcass cords, organic fiber cords such as polyester(polyethylene terephthalate, polyethylene naphtarete, fully aromaticpolyester), nylon (aliphatic polyamide), rayon and the like are suitablyused in the case of passenger car tires. But, steel cords can be usedtoo, depending on the tire size, usage and the like.In this embodiment, the carcass is composed of a single ply 6A of PETcords arranged radially at 90 degrees, and the cord count is set in arange of from 40 to 60 ends/5 cm.The carcass ply 6A extends between the bead portions 4 through the treadportion 2 and sidewall portions 3, and is turned up around the bead core5 in each of the bead portions from the inside to the outside of thetire to form a pair of carcass ply turnup portions 6 b and a carcass plymain portion 6 a therebetween. Between each of the carcass ply turnupportions 6 b and the ply main portion 6 a, there is disposed a bead apex8 made of a hard rubber extending radially outwardly from the bead core5 while tapering towards its radially outer end.

The breaker 7 is disposed radially outside the crown portion of thecarcass 6, and composed of at least two plies 7A and 7B each made ofparallel cords laid at an angle of 15 to 40 degrees with respect to thetire equator.

For the breaker cords, high-strength cords such as steel cords, highmodulus organic fiber cords such as polyethylene naphtarete(PEN),polyethylene terephthalate(PET) and aromatic polyamide and the like canbe used.

With respect to the tire equator, the cords in the radially inner ply 7Aare inclined towards one direction, and the cords in the radially outerply 7B are inclined towards the opposite direction to theabove-mentioned one direction.

The radially inner ply 7A is the widest ply of which width defines thewidth WB of the breaker 7. The radially outer ply 7B is smaller in widththan the inner ply 7A.

In this embodiment, the breaker 7 is composed of the two cross plies 7Aand 7B of steel cords, and extends. all over the width of the treadportion.

The band 9 is composed of at least one ply 9A of at least one cord 10wound spirally at an angle of not more than 5 degrees with respect tothe tire circumferential direction. The ply 9A can be a full-width plycovering the substantially entire width of the breaker 7, namely, notless than 95% of the breaker width WB, or a pair of axially spaced edgeplies each covering one of the axial edge portions of the breaker 7.Thus, the band 9 can be the full-width ply(s) only, or the axiallyspaced edge plies only, or a combination of the full-width ply(s) andthe edge plies.

In this embodiment, the band 9 is made up of a single full-width ply,and the width W of the band 9 is almost same as the width WB of thebreaker 7.

In this embodiment, in order to improve the manufacturing efficiency,the band ply 9A is formed by spiral winding a tape 13 of unvulcanizedrubber 12 in which a plurality of parallel cords 10 are embedded alongthe length thereof as shown in FIG. 3. of course it is possible to winda single cord 10 alone or a tape of unvulcanized rubber 12 in which asingle cord 10 is embedded along the length thereof. In the case of thetape 13, the number of the cords embedded therein is at most 10, usually4 to 6.

In any case, the angles of the windings of the band cord(s) are not morethan 5 degrees with respect to the tire circumferential direction inorder to improve the tire uniformity and to provide a strong hoop effectfor the tread portion 2.

The tire 1 is manufactured by first making the green tire by assemblingraw tire components including the band, and then vulcanizing the greentire in a mold as usual. During vulcanizing the green tire in the mold,a bladder put inside the green tire is inflated to press the outersurface of the green tire onto the inner surface of the mold. Therefore,although it is desirable that the band is hard to stretch in thefinished tire, the raw band has to be stretched during vulcanization inorder that the outer surface of the tread portion is fully pressed ontothe tread molding face of the mold. For that purpose, in general, about2% elongation is required for the band cord, therefore the cord modulusshould be relatively low in a cord elongation range between 0% and about2%.

To achieve such a double modulus cord as shown in FIG. 1, theload-elongation curve a of the band cord 10 has to be provided with alow-modulus zone S1 and a high-modulus zone S2 of which boundary point Pis in an elongation range of from 1.0 to 7.0%, and the nominal modulusratio EH/EL is not less than 2.0 but not more than 10.0.

If the load of the band cord 10 at 1% elongation is more than 60 N, thestretch during vulcanization becomes insufficient. If the load at 1%elongation is less than 20 N, although a sufficient stretch can beobtained, in the finished tire the hoop effect of the band is decreasedand the high speed durability tends to deteriorate. Therefore, the loadat 1% elongation is preferably set in a range of from 20 to 60 N.

If the load of the band cord 10 at 3% elongation is less than 225 N,there is a tendency that the rigidity of the tread portion becomesinsufficient and as a result the steering stability is deteriorated. Ifthe load at 3% elongation is more than 431 N, the rigidity becomesexcessively increased and the ride comfort is decreased. Therefore, theload at 3% elongation is preferably set in a range of from 225 to 431 N.

In order to reduce the weight of the band, the band cord 10 is made upof a plurality of materially identical high-strength high-modulusorganic filaments (f).

As such high-strength high-modulus organic filaments (f), for example,aromatic polyamide filaments, fully aromatic polyester filaments,polyvinyl alcohol filaments whose strength is not less than 15 g/dtex,carbon filaments, polyketone filaments, rayon filaments and the like canbe used. Among them, aromatic polyamide filaments are especiallypreferred.

If materially different organic filaments are incorporated into onecord, which usually results in coexistence of a high-modulus filamentand a low-modulus filament, thus, the load applied to the cord isunevenly shared therebetween, and one of them is first broken and thenthe other is broken as the load concentrate thereon. Thus, on the wholethe cord tends to become weak when compared with the cord according tothe present invention having the same weight.

The band cord 10 is made up of a plurality of dip-and-stretch treatedstrands 22 (hereinafter dipped strands 22). In the example shown in FIG.4, the band cord 10 is made up of two dipped strands 22. In FIG. 5showing another example, the cord 10 is made up of four dipped strands22.

The number of the dipped strands 22 is at least two, preferably at most8, more preferably 6 or less.

In any case, each of the strands 22 is formed by first-twisting a bunch21 of a plurality of the organic filaments (f).

Then, the first-twisted filament bunch 21 is subjected to adip-and-stretch treatment K.

As shown in FIG. 6, the dip-and-stretch treatment K comprises a dipprocess Ka, a drying process Kb, a stretch process Kc, and a relaxprocess Kd.

The dip process Ka is such that the first-twisted filament bunch 21 isdipped into a solution of a resin to coat the surfaces of the organicfilaments. For example, resorcinol formalin latex can be used as thesolution.

In the case of organic filaments inferior to adhesion with rubber suchas aromatic polyamide and fully aromatic polyester, it is preferablethat an additive to improve the adhesion such as epoxy compounds,isocyanate compounds, urea compounds and the like is added into thesolution.Also it is preferred to subject the bunch 21 to a two-bath dippingprocess, where first dipped into a solution or emulsion of theabove-mentioned additive to improve the adhesion and then dipped intothe above-mentioned solution of the resin.

The drying process Kb is such that the dipped filament bunch 21 is driedby heating it to a specific temperature controlled within a range offrom 100 to 160 degrees C. for 60 to 300 seconds until completely oralmost completely dried, while applying a tension of 0.1 to 1.5 g/dtex.

The stretch process Kc is such that the filament bunch 21 is stretchedby applying a specific tension in a range of 0.1 to 1.5 d/dtex forexample 0.2 g/dtex, while heating it to a specific temperaturecontrolled within a range of from 215 to 255 degrees C. for 30 to 120seconds.

The relax process Kd is such that the filament bunch 21 is graduallycooled to room temperature, while gradually decreasing theabove-mentioned applied tension.

With this dip-and-stretch treatment K, the dipped strand 22 isaccomplished. Then, directly or indirectly after reeled, the treatedstrand 22 is fed to the next process (1) or (2).

The process (1) is utilized to make the example cord shown in FIG. 4. Inthis process, the two or more strands 22 are second twisted togetherinto the final cord 10. Thus, the second twist is the last twist.

All of the strands 22 have the same first twist directions which areopposite to the second twist direction.

It is possible that all of the strands 22 have the same first twistnumbers, but in this example, in order to increase of the ratio EH/EL,the dipped strands 22 include at least two types of strands: one is alow-twist strand 22A whose first twist number N is a minimum value Nmin,and another is a high-twist strand 22B whose first twist number N is amaximum value Nmax, wherein the ratio Nmin/Nmax is preferably not morethan 0.9, more preferably not more than 0.8, but, preferably not lessthan 0.5. In this case, in view of the compatibility between the fatigueresistance of the cord and the nominal elastic modulus EH of thehigh-modulus zone, the second twist number M of the cord 10 ispreferably not less than the minimum value Nmin and not more than themaximum value Nmax. It is especially preferable that the second twistnumber M is equal to the minimum value Nmin.

As to the thickness of the strand (or the total dtex number of thefilaments therein), it is possible that all of the strands 22 have thesame thickness. But, it is also possible that the strands 22 havedifferent thicknesses. In the later case, accordingly the cord iscomposed of at least one thicker strand 22 a and at least one thinnerstrand 22 b. Preferably, the first twist number of the thinnest dippedstrand 22 b is set at the maximum value Nmax, and the first twist numberof the thickest dipped strand 22 a is set at the minimum value Nmin.This also facilities the increase of the ratio EH/EL.

The process (2) is utilized to make another example cord 10 which is, asshown in FIG. 5, made up of at least one dipped strand 22 c (in FIG. 5two dipped strands 22 c) forming a core 25, and at least one dippedstrand 22 s (in FIG. 5 two dipped strands 22 s) forming a sheathe 26around the core 25.

In this cord structure, it is preferable that the ratio DS/DC of thetotal dtex number DS of the strand 22 s forming the sheathe to the totaldtex number DC of the strand 22 c forming the core is not less than 2.3;and

the first twist number NS of the sheath strand 22 s is less than thefirst twist number NC of the core strand 22 c, and the ratio NS/Nc is ina range of from 0.07 to 0.11.

In the case that the core 25 is made up of a single strand 22 c, aplurality of strands 22 s are second twisted around the core 25.

In the case that the core 25 is made up of a plurality of strands 22 c,these strands 22 c are second twisted into the core 25. And then atleast one strand 22 s is second twisted around the core 25 in the samedirection as the second twist direction of the core. The second twistnumber M of the sheath strand 22 s is less than the first twist numberNC of the core strand 22 c and in a range of from 0.5 to 1.2 times thefirst twist number NS of the sheath strand 22 s.

In the case of the core 25 made up of a plurality of strands 22 c, thesecond twist of the core strands 22 c may be omitted substantiallybecause the sheathe 26 can prevent the core strands 22 c from loosing.

In any case, the position of the boundary point P and the value of EH/ELcan be adjusted by changing the thickness, first twist number and secondtwist number of the strands 22.

Comparison Tests

Radial tires of size 205/50R16 (rim size 16×6.511) for passenger carswere made and tested for high-speed durability and vibrationcharacteristic (uniformity).

Except for the band cords, all of the tires had the same structure shownin FIG. 2, wherein

the carcass was composed of a single ply of 1680 dtex/2 PET cordsarranged at 90 degrees with respect to the tire equator with a cordcount of 50 ends/5 cm beneath the bead cores,

the breaker was composed of two cross plies of 1×4×0.27 steel cords laidat +22 and −22 degrees with respect to the tire equator with a cordcount of 40 ends/5 cm, and

the band was composed of a full-width ply made of spirally wound cordsshown in Table 1 and 2 with a cord count of 49 ends/5 cm.

High-Speed Durability Test:

The tire inflated to 300 kPa was run under a tire load of 3.90 kN by theuse of a tire drum tester, and the running speed was increased every 20minutes at a constant step of 10 km/h in order to obtain the runningspeed at which any damage was caused on the tire resulting from theband. The test results are indicate in Tables 1 and 2 by an index basedon Ref.1 tire being 100 wherein the larger the value, the better thedurability.

Vibration Characteristic Test:

The tire inflated to 200 kPa was run at a speed of 120 km/h under a tireload of 4.00 kN by the use of a tire drum tester, and vibrationsgenerated by the rotating tire were measured on the axle.

The results are indicated in Tables 1 and 2 by an index-based on Ref.1being 100, wherein the larger the value, the better the vibrationcharacteristic.

Tire Weight:

The weight is indicate by an index based on Ref.1 being 100.

The abbreviations used in Table 1 and 2 are as follows.

In the “Material” item,

nylon: aliphatic polyamide

aramid: aromatic polyamide

rayon: rayon

vinylon: polyvinyl alcohol

vectran; fully aromatic polyester

PET: polyethylene terephthalate (aromatic polyester)

PEN: polyethylene naphtarete (aromatic polyester).

In the “Fabrication sequence” item,

-   -   A: first twist, second twist, and then dip-and-stretch treatment    -   B: first twist, dip-and-stretch treatment, and then second        twist.        In the “Status of vulcanization” item,    -   *1: The stretch of the band during vulcanizing the green tire        became insufficient and a molding defect occurred.

TABLE 1 Tire Ref. 1 Ref. 2 Ref. 3 Ref. 4 Ref. 5 Ref. 6 Ref. 7 Ex. 1 Bandcord Material nylon & nylon aramid aramid aramid aramid rayon aramidaramid Twist structure FIG. 4 FIG. 4 FIG. 4 FIG. 4 FIG. 4 FIG. 4 FIG. 4FIG. 4 Number of strands 2 2 2 2 2 2 2 2 One strand nylon Total dtexnumber 940 940 1670 1100 1100 800 1220 1100 First twist number* 30 47 3046 68 80 59 68 Other strand aramid Total dtex number 1670 940 1670 11001100 1100 1840 1100 First twist number* 42 47 30 46 52 52 48 52Nmin/Nmax 1 1 1 1 0.76 0.65 0.81 0.76 Second twist number* 43 47 30 4652 52 48 52 Boundary point (%) 4.1 non non non non non non 3.0 Modulusratio (EH/EL) 7.5 1 1 1 1 1 1 3 Fabrication sequence A A A A A A A BStatus of vulcanization no no *1 *1 *1 *1 *1 no problem problem problemTire weight 100 99.8 100.3 100.2 100.1 99.8 100.5 99.9 High-speeddurability 100 95 — — — — — 108 Vibration characteristic 100 85 — — — —— 107 Tire Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Band cord Materialaramid aramid rayon Vinylon Vectran PET PEN Twist structure FIG. 4 FIG.4 FIG. 4 FIG. 4 FIG. 4 FIG. 4 FIG. 4 Number of strands 2 2 2 2 2 2 2 Onestrand Total dtex number 800 440 1220 1130 1100 1670 1670 First twistnumber* 80 108 59 60 60 40 40 Other strand Total dtex number 1100 11001840 1130 1100 1670 1670 First twist number* 52 52 48 38 53 30 30Nmin/Nmax 0.65 0.48 0.81 0.63 0.88 0.75 0.75 Second twist number* 52 5248 38 53 40 40 Boundary point (%) 3.0 7.0 5.0 6.0 2.0 4.0 4.0 Modulusratio (EH/EL) 4 10 6 7 2 5 5 Fabrication sequence B B B B B B B Statusof vulcanization no no no no no no no problem problem problem problemproblem problem problem Tire weight 99.8 99.7 100.5 100 100 100.4 100.4High-speed durability 107 105 102 101 101 104 105 Vibrationcharacteristic 106 103 102 102 101 105 106 *turn/10 cm

TABLE 2 Tire Ex. 9 Band cord Material aramid Twist FIG. 5 Core strandnumber 2 One strand Total dtex number 440 First twist number Nc (turn/10cm) 95 Second twist number Nm (turn/10 cm) 95 Other strand Total dtexnumber 440 First twist number Nc (turn/10 cm) 95 Second twist number Nm(turn/10 cm) 95 Sheathe strand number 2 One strand Total dtex number1670 First twist number Ns (turn/10 cm) 10 Second twist number M 72(turn/10 cm) Other strand Total dtex number 1670 First twist number Ns(turn/10 cm) 10 Second twist number M (turn/10 cm) 7 Boundary point (%)1.5 Modulus ratio (EH/EL) 7 fabrication sequence B Status ofvulcanization no problem Tire weight 100.5 High-speed durability 105Vibration characteristic 106

As apparent from the comparisons between Reference tires and Exampletires, in particular comparisons of Ex.1 with Ref.5, Ex.2 with Ref.6,and Ex.4 with Ref.7—each pair being the same in the dtex number, firsttwist number and second twist number—, owning to the dip-and-stretchtreatment carried out between the first twisting process and the secondtwisting process, the Example cords were provided with the remarkabledouble modulus zones, and a necessary stretch could be obtained duringtire vulcanization, and as a result, the tires could be molded withoutproblem and displayed excellent performances. By contrast, in the caseof Reference tires 3-7, molding defects were occurred, and the tires hadno commercial value.

Aside from passenger car tires, the present invention can be applied tovarious tires as well, for example, motorcycle tires, SUV tires and thelike. For example, in the case of motorcycle tires provided with nobreaker 7, the band 9 is directly disposed on the radially outside ofthe crown portion of the carcass 6.

1. A pneumatic tire comprising a carcass extending between a pair ofbead portions through a tread portion and a pair of sidewall portions,and a band made of at least one spirally wound cord disposed radiallyoutside the carcass in the tread portion, wherein the band cord consistsof materially identical high-strength high-modulus organic filaments,and the band cord is obtainable by the following process (1) or (2): (1)the high-modulus organic filaments are divided into a plurality ofbunches, the bunches are each first twisted separately from each other,the first twisted bunches are each subjected to a dip-and-stretchtreatment, and the treated bunches are second twisted together; or (2)the high-modulus organic filaments are divided into a plurality ofbunches, the bunches are each first twisted separately from each other,the first twisted bunches are each subjected to a dip-and-stretchtreatment, some of the treated bunches are second twisted together toform a core, and at least one remaining treated bunch is spirally woundaround the core, whereby the band cord has a load-elongation curvecomprising a low-modulus zone between the origin and a boundary pointand a high-modulus zone between the boundary point and the breakingpoint, and the boundary point is at a certain elongation in a range offrom 1 to 7%, and the ratio EH/EL of a nominal elastic modulus EH of thehigh-modulus zone and a nominal elastic modulus EL of the low-moduluszone is in a range of from 2.0 to 10.0, wherein the load of the bandcord at 1% elongation is in a range of from 20 to 60 N, and the load ofthe band cord at 3% elongation is in a range of from 225 to 431 N. 2.The pneumatic tire according to claim 1, wherein the organic filamentsare aromatic polyamide filaments, fully aromatic polyester filaments,polyvinyl alcohol filaments having a strength of not less than 15g/dtex,carbon filaments, polyketone filaments or rayon filaments.
 3. Thepneumatic tire according to claim 1, wherein said at least one spirallywound cord of the band forms a single layer extending across the almostentire width of the tread, and the carcass is composed of a single plyof organic fiber cords arranged radially at substantially 90 degreeswith respect to the tire equator.
 4. The pneumatic tire according toclaim 1, wherein the first twisted bunches include a low-twist bunch ofwhich first twist number is a minimum value Nmin, and a high-twist bunchof which first twist number is a maximum value Nmax, and the ratioNmin/Nmax is not more than 0.9.
 5. The pneumatic tire according to claim4, wherein the second twist number M of the treated bunches is not lessthan the minimum value Nmin and not more than the maximum value Nmax. 6.The pneumatic tire according to claim 1, wherein all of the firsttwisted bunches have same first twist numbers.